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Myoglobin is a cytoplasmic hemoprotein, expressed solely in cardiac myocytes and oxidative skeletal muscle fibers, that reversibly binds O2 by its heme (iron-containing porphyrin) prosthetic group in the center around which the remaining apoprotein folds. The red color that meat takes is partly determined by the charge of the iron atom in myoglobin and the oxygen attached to it. When meat is in its raw state, the iron atom has a charge of +2 and is bound to an oxygen molecule. Myoglobin is a single-chain globular protein of 153 amino acids and has a molecular weight of 16,700 daltons. Unlike the blood-borne hemoglobin, to which it is structurally related, this protein does not exhibit cooperative binding of oxygen, since positive cooperaivity is a property of multimeric/oligomeric proteins only. Recent gene disruption studies using the myoglobin deficient mouse model support the conclusion that the functions of myoglobin include: facilitated oxygen transport, the storage of oxygen and a scavenger of nitric oxide or reactive oxygen species. Myoglobin is an early indicator of cardiac ischemia that achieves maximal sensitivity 3 to 4 hours after symptom onset. However, it is only marginally more sensitive than CK-MB early after the onset of symptoms, and it is less sensitive 6 to 8 hours after symptom onset. Because of rapid clearance from the blood, myoglobin may "miss" late-presenting patients, and it is less cardiospecific than CK-MB.